Limit abm that plants new fungi.

abms.lua

@@ -679,15 +679,14 @@ minetest.register_abm({
 	end
 })
 
--- ***********************************************
--- new fungi -- This may be too cpu-intensive.
+-- new fungi
 local mushroom_nodes = {}
 for _, mushroom in pairs(mushrooms) do
 	mushroom_nodes[#mushroom_nodes+1] = {name = mushroom}
 end
 minetest.register_abm({
 	nodenames = {"default:dirt"},
-	neighbors = {"air"},
+	neighbors = {'fun_caves:stone_with_lichen', 'fun_caves:stone_with_algae'},
 	interval = time_factor,
 	chance = 300,
 	catch_up = false,
@@ -708,7 +707,6 @@ minetest.register_abm({
 		end
 	end
 })
--- ***********************************************
 
 -- mushroom growth -- small into huge
 local giant_mushroom_stem_node = {name = 'fun_caves:giant_mushroom_stem'}